Silicon nitride has generated considerable interest as a ceramic material because of its high thermal and oxidative stability and extreme hardness. Other advantageous properties include low electrical conductivity, low coefficient of thermal expansion, excellent thermal shock and creep resistance, high strength at elevated temperatures and excellent corrosion resistance.
One recently developed method for obtaining silicon nitride-containing ceramic materials is the pyrolysis of polyorganosilazanes. Seyferth et al. (U.S. Pat. No. 4,482,669) describe the base-catalyzed cross-linking of the ammonolysis product of an organodihalosilane to form a polysilazane ceramic precursor. This material is especially useful as a binder for ceramic powders. Verbeek (U.S. Pat. No. 3,853,567) produced shaped forms such as fibers comprising a homogeneous mixture of silicon carbide and silicon nitride by pyrolyzing a shaped form of a fusible carbosilazane resin in an inert atmosphere. Lebrun and Porte (U.S. Pat. No. 4,689,252) describe the preparation of polysilazanes by the platinum-catalyzed hydrosilation reaction of a silazane or a mixture of silazanes that contain both Si-H and unsaturated hydrocarbon groups. These polymers can be cross-linked by heating and can be used for coating or impregnating substrates with ceramic material after pyrolysis. Laine and Blum (U.S. Pat. No. 4,612,383) describe the catalytic activation of Si-H, Si-Si or Si-N bonds in silanes or silazanes by metal complexes to give oligomers and polymers. The products can be pyrolyzed to silicon nitride. King et al. (U.S. Pat. No. 4,675,424) describe the preparation of polysilazanes by the reaction of an aminosilane with a low molecular weight amine in the presence of an acid catalyst. Such polymers can be pyrolyzed under nitrogen to give silicon nitride-containing ceramics. Porte and Lebrun (U.S. Pat. No. 4,722,988) disclose the preparation of polysilazane precursors for ceramics by cross-linking silazanes containing alkenyl or alkynyl substituents in the presence of a free radical generator such as a peroxide. Fink (U.S. Pat. No. 3,239,489 describes the preparation of polyureasilazanes having no mobile hydrogen atoms by the reaction of di- or polyfunctional isocyanates with silazanes. Pyrolysis of these polymers to ceramic materials is not disclosed.
In general, the above methods are deficient in that it is difficult or impossible to control the viscosities of the polysilazanes so that they are suitable for the intended end use of the polymer. For example, low viscosities are desirable for polymers used to produce thin films or to infiltrate porous ceramic bodies, and high viscosities are desirable for making fibers.